Process for manufacturing a fermented milk

ABSTRACT

The invention relates to a manufacturing process of stirred or drinking fermented milk or fresh cheese comprising a smoothing step after fermentation wherein said smoothing step is performed by a ring-shaped rotor-stator mixer, each ring of the rotor and the stator being provided with radial slots having a given width, and adjusting the rotational speed to adjust the peripheral velocity.

The invention relates to a process for the manufacturing of a fermentedmilk, namely a stirred or drinking fermented milk or fresh cheese,comprising a smoothing step after fermentation in a tank to generate anhomogeneous and smooth texture.

A stirred fermented milk, is incubated in a tank and the final coagulumis broken by stirring prior to cooling and packing. The texture issomewhat like a thick cream and is less firm than that of a set yoghurt(which is incubated and cooled in the final package and is characterizedby a firm jelly like structure). A drinking fermented milk is verysimilar to a stirred fermented milk, but its viscosity is much lower.

The stirred operation after fermentation is a key process in themanufacturing of stirred or drinking fermented milk, such as a stirredyoghurt or a drinking yoghurt. This unit operation is usually performedby using filters or valves.

More particularly, the process also relates to a fermented milk that hasbeen submitted to a step of high pressure homogeneization beforefermentation.

The first known solution for the stirring operation after fermentationis the continuous agitation of the fermented mass in the tank during thetransfer operation to cooler. But the standard agitation in a tank leadsto a high viscosity loss.

The second solution, namely using a static filter has been a betteralternative to smooth the product, but the new ingredient development,some texture innovation, the complexity of current lines for a big mixof different products and the viscosity target variation, require a newmore sensible system for this operation.

On the other hand, the stirred yoghurt manufacturing with a staticfilter is impossible to do without changing filter during productionbecause of the plugging.

A hand cleaning of the filter is necessary to achieve the totalcleanliness with hygiene risks for the products. It also impliesinterrupting the production during the cleaning operation.

The present invention relates to a process for the manufacture ofstirred or drinking fermented milk or a fresh cheese comprising after afermentation step a smoothing step wherein said smoothing step isperformed by a rotor stator mixer comprising a ring shaped rotor and aring shaped stator, each ring of the rotor and of the stator beingprovided with radial slots having a given width, comprising adjustingthe rotational speed of the rotor to adjust the peripheral velocity.

The stator head may have three rings and the rotor head three rings.

The radial gap between rings of the stator and of the rotor may bebetween 0.5 mm and 2 mm.

The slot width may be between 0.3 mm and 2 mm, and more particularlybetween 0.5 mm and 1.8 mm

The rotor may be operated so that the peripheral velocity is not higherthan 16 m/s for a stirred fermented milk, more particularly between 3.5m/s and 16 m/s or between 5.5 m/s and 11.4 m/s (depending on the lineflow rate and the machine model).

For a stirred fermented milk (e.g. stirred yoghurt) or a fresh cheese,with a target viscosity between 300 mPas and 3700 mPas, the peripheralspeed is between 3.5 m/s and 16 m/s.

For a flow rate between 150 l/h and 20,000 l/h, the peripheral speed ispreferably between 3.8 m/s and 15.7 m/s.

For a flow rate between 20,000 l/h and 60,000 l/h, the peripheral speedis preferably between 5.5 m/s and 11.4 m/s.

For a drinking fermented milk (e.g. a drinking yoghurt) with a targetviscosity between 30 mPas and 300 mPas, the peripheral velocity isbetween 22 m/s and 30 m/s for a flow rate between 150 l/h and 20,000 l/hand preferably between 25 and 30 m/s for a flow rate between 20,000 l/hand 60,000 l/h. With such a low viscosity, these speeds do not bring ahigh shear rate to the product, hence minimizing the viscosity loss.

The fermented milk may be of the fat free type and the process comprisesadjusting the peripheral velocity.

The fermented milk may be of a medium fat formula comprising a fatcontent between 3% and 5% in weight and the process comprises adjustingthe peripheral velocity.

The fermented milk may have a fat free formula with an addition ofstarch between 1.5% and 3% in weight and the process comprises adjustingthe peripheral velocity, the slot width being less than 1 mm, or, with aslot width between 0.3 mm and 0.8 mm, adjusting the peripheral velocityup to 11 m/s.

The fermented milk may have a high-fat content between 7.5% and 10% inweight.

The fermented milk may be a fresh cheese and the process comprisesadjusting the peripheral velocity, the slot width being between 1 mm and1.5 mm.

The invention also relates to a smoothing rotor-stator mixer forperforming the process defined above, comprising a ring shaped rotorhead and a ring shaped stator, each ring of the rotor and of the statorbeing provided with radial slots having a given width.

The stator may have three rings and the rotor head may have three rings.

The slot width may be between 0.3 mm and 2 mm and more particularlybetween 0.5 mm and 1.5 mm.

A radial gap between the rings of the stator and of the rotor may bebetween 0.5 mm and 2 mm.

The rotational speed of the rotor may be adjustable so that theperipheral velocity is not higher than 16 m/s, and more particularlybetween 3.5 m/s and 16 m/s.

In the appended drawing,

FIG. 1 illustrates the smoothing step according to the invention,

FIGS. 2 a and 2 b show the internal structure of the mixer and

FIGS. 3 a and 3 b illustrate a test for a formula that is fat free withstarch.

It is worth mentioning that in prior art, there was a great variety ofmachines for mixing or dispersing the ingredients in the first stage ofthe process (ingredient mixing).

The current fields of application of these machines are homogeneousmixing, suspending and dissolving powders, dispersing applications,emulsifying.

No one of these machines may be used for the smoothing of yoghurts afterfermentation, since a direct implementation of such an equipment wouldproduce a high or very high viscosity loss that would not be acceptable.

According to the invention, it has been found that a machine of therotor-starter type having ring shaped stator and rotor each havingradial slots could be adapted to achieve the smoothing operation whichminimizes the loss of texture, i.e. the loss of viscosity.

State of the art machine of this type always provide high shear ratesbecause on the one hand of the dimensioning of the stator and rotor andon the other hand of their fixed frequency of rotation (50 Hz or 100 Hz,i.e. 3000 or 6000 rpm), corresponding to a range of velocities of 18-25m/s. This range of velocities is not suitable for the process accordingto this invention for products such as a stirred or drinking fermentedmilk, or a fresh cheese.

The process according to the invention concerns a fermented milk asdefined by the Codex Alimentarius Standard for Fermented Milks (CODEXSTAN 243-2003) or a fresh cheese. The preferred product is a stirred ordrinking yoghurt (the meaning of the word “yoghurt” being the broadestmeaning we could have—i.e. the U.S. meaning). Yoghurt according to theinvention is including product containing some bacterial strains likeLactobacillus spp. paracasei, Bifidobacterium aninalis subsp lactis,Lactococcus spp. lactis, Lactobacillus spp. plantarum . . . and productcontaining vegetable oils like phytosterols (and sterols esters) orPUFA.

Cheese according to the invention is the unripened semi-solid product inwhich the whey protein/casein ratio does not exceed that of milk,obtained by:

a) coagulation wholly or partly the following raw materials: milk,skimmed milk, partly skimmed milk, cream, whey cream, or buttermilk, orany combination of these materials, through the action of rennet orother suitable coagulating enzymes, and by partially draining the wheyresulting from such coagulation; and/or

b) processing techniques involving coagulation of milk and/or materialsobtained from milk which give an end-product with similar physical,chemical and organoleptic characteristics as the product defined undera).

Fresh cheese may be obtained by adding rennet to a milk mass,fermenting, and draining by centrifugation to obtain an homogeneouspaste which may be smoothed according to the present invention.

The product used in the experiments is a yoghurt-based fermented whitemass, manufactured with different fat and protein content in the rangeof 0-10% Fat Content FC and 3-5.5% Protein Content PC (in weight), and1.5-3% starch content for a formula including starch.

The products have been classified in four different categories:

Fat Free Formula: FFF (less than 0.5% FC)

Medium Fat Formula: MFF (between 3% and 5% FC)

FF with starch & gelatine: FFS Formula (less than 0.5% FC with 1.5-3%starch in weight).

High-Fat Content Formula: HFC (9.5% FC) for yoghurt or for fresh cheese.

The corresponding specifications and components are gathered in Table 1.

TABLE 1 Formula specifications for the white masses White Fat ProteinDry masses (%) (%) weight (%) Range 0-10 3-5.5 FFF 0.05 4.90 13.70 MFF4.00 4.40 20.85 FFS 0.07 4.02 10.97 HFC 9.6 4.35

For the FFS formula, tested compositions comprise 2.2% starch and 0.2%gelatine in weight.

Moreover, experiments has been made with white mass used for makingfresh cheeses with a fat content between 3.4 and 7.1% and a proteincontent between 4.9 and 5.4%.

FIG. 1 illustrates the smoothing stage according to the invention. Apump 2 is placed downwards from a fermentation tank 1. The in-line mixer10 is placed after the pump. The product is recovered in a tank 20 atthe output of the in-line mixer 10.

For the experiment design, the fermented product is pumped, and thensmoothed in the pump 2 at about 38-39° C. (depending on the culture), atthe pH target of 4.65.

Sampling is done on the product at 38-39° C. after the smoothingoperation. All different products are maintained at fermentationtemperature until batch experiments are finished. Then, the fermentedmilk is packaged and cooled into a cooling cell down to 10° C.

The samples are stored at 10° C. prior to analysis.

FIG. 2 a shows the internal structure of the stator head of the mixer 10and FIG. 2 b shows the assembly of a rotor-stator head of the mixer 10.The stator head 3 is comprised of three rings 4 each of which isprovided with radial slots 5. The rotor head 6 is comprised of threerings 7 each of which is provided with radial slots 8. Radial slots 5and 8 have a width Ws and the stator-rotor gap between the stator rings4 and the rotor rings 7 is designated by G.

FIGS. 3 a and 3 b illustrate the impact of peripheral velocity (V) onFFS Formula viscosity at D1 for a ring slot width of 1.5 mm (a) and 0.5mm (b) respectively,

The middle curve of FIG. 3 a shows the impact of the peripheral velocityon viscosity at D1 for a low radial velocity (low flow rate Q=3010 kg/h& high slot width=1.5 mm): the increase in peripheral velocity induces aviscosity loss.

The middle curve of FIG. 3 b shows the peripheral velocity impact onviscosity at D1 for a high radial velocity (high flow rate Q=5000 kg/h &low slot width of 0.5 mm): the increase in peripheral velocity allows toraise the texture until 11 m/s (maximum in 2108 at 11 m/s).

In order to obtain the same viscosity values with different parametersof flow rate (Q) and ring slot width (W), it is necessary to adjust theperipheral velocity.

The resident time in mixer 10 is close to a few seconds. The two maincomponents of the velocity field are the peripheral velocity (flowbetween rotor-stator gap) and the radial component (flow along the ringslots).

The peripheral velocity depends on the rotational speed of the rotorhead. On the other hand, the radial velocity depends on both the flowrate and on the geometrical parameters of the rotor-stator design (slotwidth).

The object of the smoothing operation according to the invention is toobtain a smooth and possible grainless texture with a particular targetviscosity.

By adapting the geometry and the rotational speed of a rotor-statormixer, it is possible to obtain a dynamic smoothing ensuring a slowstirring of a stirred fermented milk, e.g. a yoghurt.

The adjustment of the peripheral speed allows to adjust the viscosity ofthe product and/or to monitor the viscosity in real time duringproduction.

A fermented milk according to the invention may be submitted to a mixingstep (very slow stirring) in the fermentation tank just sufficient toavoid the building of a firm jelly-like structure like that of aconventional set yoghurt. Then the process according to the inventionmay be considered as involving a double stirring (very slow stirring inthe tank, slow stirring or smoothing downstream of the tank).

Conversely, prior art filters are neither flexible nor suitable forhigh-texture white masses and lead to a rapid filter plugging and theobtained fermented milk still has grains. Smoothing with a disk filteris not suitable to achieve a smooth product. With such filters, a milkenrichment in concentrated powder of protein and/or cream would bringdrawbacks. Conventional stirring in the tank also leads to a highviscosity loss.

It is not possible to obtain new textures or to use new ingredients withthe known devices and to achieve a quality product, in viable conditionstechnically and economically.

Experimental Data:

Factors and Levels

The dynamic device includes only one rotor-stator generator with 3 ringsand a fixed gap between rotor and stator rings, of 0.5 mm. Theseparameters (1 generator, 3 rings stator and 3 rings rotor) wereoptimised on the first part of study.

On the second part, two different models of equipment were used (z66 & z120), in order to define the device size depending on the flow rate (thefirst important factor).

Three factors are essential: the flow rate (Q), the peripheral velocity(V) which depends on the rotational speed of the rotor head, and thering slot width (W_(s)) to obtain a high quality product, i.e. ahigh-texture, smooth and grainless product.

Measurements

Dynamic viscosity measurements were conducted with the rheometer RheolabMCI (Physica) at day 1 (D1) and day 15 (D15). Experiments were performedat 10° C. The imposed shear rate was 64 s⁻¹. Data at 10 s were recorded.

The smoothing operation in the rotor-stator system is a double stirringprocess with two main components of the velocity field: the peripheralvelocity (flow between rotor-stator gap) and the radial component (flowin the ring slots).

The best results for the quality product are reached by using a lowrotational speed (corresponding to a peripheral velocity up to 16 m/s)in all cases having a high-viscosity (>300 mPas), e.g. stirred fermentedmilk or fresh cheese.

Each product requires a different velocity value, and the responses interms of quality (viscosity) are different depending on the product.

The results on smoothing five different white masses with the help ofthe same rotor-stator mixing device show that:

1) FF Formula

The peripheral velocity is the most influent parameter. Its relativeweight on the viscosity response is so important that all other factorsare negligible.

To reach high-texture products, the peripheral velocity has to beadjusted according to the flow rate.

In these conditions, to reach a target viscosity of 1100 mPas, theperipheral velocity must be less than 12 m/s.

The ring slot width is the second most important factor. It has apositive effect on viscosity. An optimum is reached at 1 mm due to ahigh quadratic effect.

The flow rate has a low impact on viscosity, but the model definitiondepending on it, is also very important.

2) MF Formula

The peripheral velocity is always the parameter that must be adjustedfirst to achieve the highest textured fermented milk with the highestcreamy and cosmetic perception. As for FF formula (without fat), theperipheral velocity has a negative impact on texture.

In addition, there is a high interaction between flow rate and ring slotwidth. The ring slot width is more important as the flow rate is high.Finally, at high flow rate, the slot width should be the highest inorder to obtain products with high viscosity at D1.

The flow rate has low impact on viscosity response compared to the otherfactors.

The model (depending on the flow rate) and the slot width areestablished to minimized the radial shearing that would affect viscosityin a negative way. Once the equipment is defined, the final viscosity ofthe product is determined by the peripheral speed.

3) FFS Formula

All the factors have an impact on the viscosity response at D1, i.e. theflow rate, the peripheral velocity, the slot width, their interactionsand quadratic effects. The optimum in viscosity response depends on theratio between the two main components of the velocity that characterisethe fluid flow (peripheral and radial velocities), and so thecorresponding components of the shear rates and associated fluidparticle resident times.

As the flow rate increases and/or the slot width decreases, thecorresponding shear rates increase and so the viscosity decreases, allfactors being equal in other respects.

The peripheral velocity has a negative impact on texture properties aslong as its level is higher than that of radial velocity.

For a low radial velocity (FIG. 3 a), i.e. low flow rate and high slotwidth, the increase of the peripheral velocity allows to lower thetexture of stirred fermented milk (diminution of viscosity).

On the other hand, for a high radial velocity (FIG. 3 b), the increaseof the peripheral velocity allows to raise the texture (increase ofviscosity) of stirred fermented milk until V=11 m/s.

4) HFC Formula or Fresh Cheese Formula

With a dynamic system, as for a static one, the fat and the proteincontents have a positive impact on the product texture, the proteinbeing the most important factor for texture improvement.

Subject to either a static or a dynamic smoothing, the overall behaviourof each formula depends on its microstructure, i.e. the protein networkcohesion. The flow in an in-line rotor-stator mixing device mainlydepends on the initial viscosity of the white mass and also on themicrostructure of the white mass.

As shown in Table 2 below, a surprising texture improvement can be madeby increasing the velocity of the rotor head for some formulae (see alsoFIGS. 3 a and 3 b).

TABLE 2 Impact of the increase of the rotational rotor speed on thefermented milk texture Fat Protein Rotational speed VISCO (%) (%) (rpm)D1 (mPas) 9.57 4.35 2142 1745 4182 3713 0.09 3.19 2142 485 2730 712 6.735.4 2142 2790 4017 2903 4.83 4.32 2142 834 3858 1091 2.34 5.42 2142 13214097 534 7.43 3.06 2142 1566 3011 980 4507 604

For both low (i.e. Fat<5.2% and Protein<3.7%) and high textured product(i.e. Fat>4.8% and Protein>5.2%), the viscosity at D1 obtained with anin-line rotor-stator mixing device is higher than that obtained with aprior art disk filter. For these fermented milk formula, dynamicsmoothing is less destructive for the texture.

From these results, one can conclude that the fluid flow into therotor-stator head highly depends on the initial viscosity of theproduct. For low viscous fluid (FF formula), the flow would mainly takeplace in the rotor-stator gap. As a result, the peripheral velocity andso the corresponding components of the shear rates have a high impact onthe final product texture. The imbalance between the flow into therotor-stator gap and into the ring slot width is more important as theperipheral velocity (i.e. the rotational speed of the rotor head), ishigh in view of the radial velocity (i.e. the flow rate). Moreover, thisimbalance allows to smooth the product well and remove its grains. Itmeans that there is a mean shear rate threshold allowing to optimise theproduct aspect (smoothness, grains quantity).

The higher the viscosity of the fluid, the more laminar the fluid flowleading to reduce the imbalance between both flows. The “productsensitivity” to the radial velocity (i.e. flow rate) will be moreimportant. As a result, the influent factors on texture loss are theflow rate and the slot width (FFS formula).

The new smoothing technology is a suitable solution for reaching smoothproducts whatever the fat and the protein content, and possibly forremoving grains, which results in a smoother and more creamy perception.

The inline rotor-stator mixing device is a highly flexible equipmentallowing to enhance the product texture compared to a static filter byadjusting the rotational speed of the rotor, the other on lineparameters have been set (device model depending on the flow rate andslot width).

It is possible to adjust the viscosity of fermented milk or yoghurtmasses by a rotor-stator system, simply by fixing a particularperipheral speed which is obtained by adjusting the rotational speed ofthe mixer.

The dimensions of the equipment depend, as shown above on the productand on the target value of viscosity, generally implying a low loss ofviscosity.

Low rotation speeds (less than 16 m/s) are used for stirred fermentedmilk, e.g. stirred yoghurt, or fresh cheese.

High rotation speed (more than 22 m/s) are used for drinking fermentedmilk, e.g. drinking yoghurt.

1. A process for the manufacture of a fermented milk, namely a stirredor drinking fermented milk or fresh cheese, comprising after afermentation step a smoothing step wherein said smoothing step isperformed by a rotor stator mixer comprising a ring shaped rotor headand a ring shaped stator head, and wherein a radial gap between rings ofthe stator and the rotor is between 0.5 mm and 2 mm, and preferablyequal to 0.5 mm, each ring of the rotor and of the stator being providedwith radial slots having a given width, said process comprisingadjusting the rotational speed of the rotor to adjust the peripheralvelocity.
 2. A process as in claim 1, wherein the stator head has threerings and the rotor head has three rings.
 3. A process as in claim 1,wherein the product is a stirred fermented milk or a fresh cheese andwherein the rotor is operated so that the peripheral velocity is between3.5 m/s and 16 m/s.
 4. A process as in claim 3, wherein the flow rate isless than 20,000 l/h and wherein said peripheral velocity is between 3.8m/s and 16 m/s.
 5. A process as in claim 3, wherein the flow rate isbetween 20,000 l/h and 60.000 l/h and wherein said peripheral velocityis between 5.5 m/s and 11.4 m/s.
 6. A process as in claim 5, whereinafter smoothing, the viscosity is between 300 mPas and 3,700 mPas.
 7. Aprocess as in claim 1, wherein the product is a drinking fermented milkand wherein said peripheral velocity is between 22 m/s and 30 m/s.
 8. Aprocess as in claim 7, wherein the flow rate is less than 20,000 l/h. 9.A process as in claim 7, wherein the flow rate is between 20,000 l/h and60,000 l/h and wherein said peripheral velocity is between 25 and 30m/s.
 10. A process as in claim 7, wherein after smoothing the viscosityis between 30 mPas and 300 mPas.
 11. A process as in claim 1, whereinthe slot width is between 0.3 mm and 2 mm.
 12. A process as in claim 11,wherein the slot width is between 0.5 mm and 1.8 mm.
 13. A process as inclaim 1, wherein said fermented milk is of the fat free type.
 14. Aprocess as in claim 1, wherein said fermented milk is a medium fatformula having a fat content between 3% and 5% in weight.
 15. A processas in claim 1, wherein said fermented milk is a high fat content formulahaving a fat content between 7.5% and 10% in weight.
 16. A process as inclaim 1, wherein said fermented milk is a fat free formula with anaddition of starch between 1.5% and 3% in weight and wherein said slotwidth is between 1 mm 1 mm and 2 mm.
 17. A process as in claim 1,wherein said fermented milk is a fat free formula with an addition ofstarch between 1.5% and 3% in weight and wherein the process comprises,with a slot width between 0.3 mm and 0.8 mm, adjusting the peripheralvelocity up to 11 m/s.
 18. A process as in claim 1, wherein saidfermented milk is a fresh cheese formula, wherein the slot width isbetween 1 mm and 1.5 mm.
 19. A system for performing the processaccording to any of the preceding claims, comprising a smoothingrotor-stator mixer having a ring-shaped rotor head and a ring shapedstator head, wherein a radial gap between rings of the stator and of therotor is between 0.5 mm and 2 mm, and preferably equal to 0.5 mm, eachring of the rotor and of the stator being provided with radial slotshaving a given width, and means for adjusting the rotational speed ofthe rotor, and also comprising a link to a fermented milk tank.
 20. Asystem according to claim 19, wherein the stator head has three ringsand the rotor head has three rings.
 21. A system according to claim 19,wherein the slot width is between 0.3 mm and 2 mm, more particularlybetween 0.5 mm and 1.8 mm.
 22. A system according to claim 19, whereinthe rotational speed of the rotor is adjustable so that the peripheralvelocity is not higher than 16 m/s, and more particularly between 3.5m/s and 16 m/s.
 23. A system according to claim 19, wherein therotational speed of the rotor is adjustable so that the peripheralvelocity is between 22 ml/s and 30 m/s and more particularly between 25m/s and 30 m/s.